1. Field of the Invention
The present invention relates to a spread-spectrum communication method and apparatus.
2. Description of the Related Art
In a time-sharing communication method for converting data to bursts, in order to receive and demodulate data bursts it is necessary to establish synchronization with the data bursts to be received. It is also necessary that only desired information addressed to the receiving end be detected and extracted from the received signals. In addition, conflict between one station and another station must be avoided. Accordingly, in general, in such a communication method data is transmitted in accordance with a regular format.
FIG. 1 shows an example of a format for a data burst in the above communication method. For example, data bursts each include a preamble consisting of a synchronization code (SY), a unique word (UW) and a station-identification code (ID), and data (DA). Between the data bursts there is a guard time (GT).
A receiving end which receives the data bursts uses the synchronization code in the preamble period to perform reproduction of a carrier, input of automatic gain control (AGC), establishment of clock synchronization, and so forth. The receiving end further detects the unique word (UW) and the station-identification code (ID), and when it perceives that the successive data (DA) is desired data addressed to itself, it holds a reproduced carrier, AGC, clock synchronization and so forth until the data terminates, and it demodulates the data.
However, this communication method causes an error in the reference clock frequency between the transmitting and receiving ends. Thus, with the lapse of time, the receiving end's clock which has held the established synchronization in the preamble, also has increased synchronization errors with respect to the transmitting end's clock. In addition, for example, if the transmission line is wireless, communication quality may vary with time, which results in the possibility of the input AGC held in the preamble losing its optimum condition with the lapse of time. According to this communication method, the maximum time during which data can be transmitted with one data burst is limited by the time during which synchronization precision, AGC precision and so forth can be maintained.
According to the above communication method, when a large amount of digital data (e.g., image data or the like) adapted for recent multimedia applications is transmitted, as shown in FIG. 2, data is divided into a plurality of portions, and the complete data must be transmitted as a plurality of data bursts. As a result, the preamble time and the guard time with respect to the time used for transmission of the actual data increases, and there is the possibility of an interrupting burst from another station occurring between the divided data bursts. Consequently, the data throughput deteriorates.
In order to extend the maximum time during which the data can be transmitted with one data burst, a precise frequency oscillator, and a complicated synchronization circuit or AGC must be used, which disadvantageously requires an expensive, large-sized apparatus.